The RET proto-oncogene encodes a receptor-type tyrosine kinase, which serve as a signaling component for the receptor complex of glial cell-line derived neurotrophic factor or neurturin. In human papillary thyroid carcinoma (PC), RET is activated by somatic arrangements with different genes, generating RET/PTC oncogenes. In each case, the intercellular domain of RET, which has tyrosine kinase activity, is fused to the N- terminus of the activating gene that is capable of dimerization. Our long- term goal is to identify and characterize RET/PCR-induced cellular changes and signaling pathways that contribute to the development and the pathological properties of PC. We have identified three distinctive, early cellular changes in the thyroid glands of our Tg-PTC1 transgenic mice, which express RET/PTC1 under the control of the thyroglobulin (Tg) promoter. In the current proposal, three specific aims are identified. In Aim 1, we seek to determine whether cellular changes are direct effects of RET/PTC1 by investigating the temporal and dosage relationships between RET/PCT1 expression and these cellular changes in the thyroid glands of Tg-PTC1 transgenic mice, and in primary cultured porcine thyrocytes as a system more amenable to manipulation. In Aim 2, we will determine whether pY294, pY404, and pY451-mediated signaling pathways are responsible for these distinctive cellular changes, and which of these pathways are essential for RET/PTC1 to induce thyroid tumors with many characteristics of PC. We propose to initially characterize these essential signaling pathways by identifying the signaling proteins that bind to RET/PTC1 and the differentially expressed genes induced by RET/PTC1 in cultured thyrocytes. In Aim 3, we will compared thyroid tumorigenicity, cellular changes, and signaling pathways induced by RET/PTC3 with RET/PTC1 to address the biological significance and the clinical relevance of RET/PTC3 compared to RET/PTC1.